Treatment of the Conformational Contributions in Quantum Mechanical Calculations of the Redox Potentials of Nitroxyl Radicals

The redox potentials of cyclic nitroxyl radicals have been calculated for the reaction pair R2N+=O/R2N-O-center dot using density functional theory and the polarizable continuum solvent model. The calculations of the redox potentials have taken into account the free-energy contributions from conformational ensembles of reactants using three computational schemes that included a thermodynamic cycle model for electron transfer in the gas phase and solution as well as direct calculations of the equilibrium free energies in water. Correlations between the experimental data and redox potentials computed by the three methods have been explored. The smallest inaccuracies were found for the approaches that use solute structures optimized in water.